1. Field of the Invention
The present invention relates to a lubricating oil composition comprising phosphorus compounds having a particular structure and having excellent lubricity. Further, it relates to a working fluid composition for refrigerating machines using the lubricating oil composition.
2. Discussion of the Related Art
With lengthened intervals of oil changes, need of energy-saving, use of high performance machines, and down-sizing of machines, demand for the performance of lubricating oils has become severe. In particular, lubricating oils with a high thermal stability and a high oxidation stability have been strongly sought. In the situation where there has been increasing public concern about global environmental pollution, such as depletion of ozone layer caused by flon, the earth warming due to carbon dioxide and methane, destruction of forests by sulfurous acid gas and NO.sub.x in exhaust fumes, and pollution of soil and lakes due to chemical leakage, environmental protective problems have been strongly sought also in the field of lubricating oils.
In order to meet the requirements for high thermal stability and oxidation stability, ethers, such as polyalkylene glycols, and esters, such as aliphatic diesters and hindered esters, have been developed.
In addition, from the viewpoint of the depletion of the ozone layer caused by flon, the refrigerant is changed from a chlorofluorocarbon (CFC) or a hydrochlorofluorocarbon (HCFC) to a hydrofluorocarbon (HFC). As to the refrigeration oil, a polyalkylene glycol, a hindered ester, or a carbonate, which is compatible with the hydrofluorocarbon, has been used.
However, since an oxygen-containing compound, including the ester, the polyalkylene glycol, the carbonate, or the like has a higher polarity than that of a hydrocarbon compound, including a mineral oil, or the like, the oxygen-containing compound has an excellent adsorptivity to metal surfaces. Therefore, there have been causing such problems that adsorption of such additives as oiliness improvers, antiwear additives, and extreme pressure additives is inhibited, thereby lowering the effects of these additives.
In particular, in a working fluid composition for refrigerating machines comprising an oxygen-containing compound, including a non-chlorine-containing HFC, an ester, or the like, since the lubricity improvement effects owing to hydrogen chloride generated by decomposition of CFC or HCFC, each containing a chlorine atom or chlorine atoms, cannot be expected, a further excelled lubricity is demanded in the refrigeration oil. However, there arises such a problem that triphosphates or triphosphites which have been conventionally used in refrigeration oils do not exhibit their effects in the oxygen-containing compounds.
In order to solve those problems, Japanese Patent Laid-Open Nos. 4-28792 and 4-100894 disclose the use of acid phosphoric esters and acid phosphorous esters. However, since those phosphorus compounds are acidic, there arise such problems that metals are rather corroded, and that hydrolysis of an ester or a carbonate, used as a base oil, is accelerated.
On the other hand, as to additives in the lubricant field, there have been proposed the use of neutral phosphoric esters containing hydroxyl groups. For instance, Japanese Patent Laid-Open No. 57-164192 discloses the use of a dialkyl 2-hydroxyalkyl phosphonate; Japanese Patent Laid-Open No. 60-94988 discloses the use of diester of phosphorous acid; Japanese Patent Laid-Open No. 5-302093 discloses the use of dioctyl hydroxymethylphosphonate; and Proceedings of JAST Tribology Conference (Tokyo, May, 1995), Japanese Society of Tribologists, discloses the use of a hydroxyalkyl phosphate. Each of those compounds has an alkyl chain having one hydroxyl group.
However, in the Proceedings of JAST Tribology Conference (Tokyo, May, 1995), Japanese Society of Tribologists, Minami et al. have reported that the phosphorus compound having an alkyl chain having one hydroxyl group is not effective in improving lubricity in a lubricating oil having high polarity, such as an ester oil (Proceedings 2A1-1).
Accordingly, it is an object of the present invention to provide a lubricating oil composition particularly having excellent lubricity even in the case where a base oil having a high polarity is used, and being free from metal corrosion by the additives.
Another object of the present invention is to provide a working fluid composition for a refrigerating machine containing the lubricating oil composition.
These and other objects of the present invention will be apparent from the following description.